In general, the multi-shift mechanism of an automatic transmission is implemented with a combination of a plurality of planetary gear sets, and a power train having such a plurality of planetary gear sets combined therein receives rotation power from a torque converter, changes the rotation power, and transfers the changed rotation power to an output shaft.
As the number of shift stages increases, the automatic transmission may be designed to have a more proper transmission gear ratio, which makes it possible to implement a vehicle with excellent power performance and fuel efficiency. Thus, research has been steadily conducted to implement an automatic transmission having a larger number of shift stages.
Furthermore, although automatic transmissions having an equal number of shift stages are applied, power trains thereof may have a different durability, power transmission efficiency, and size depending on a method of combining planetary gear sets. Thus, attempts have been made to develop a more reliable and compact power train which can reduce power loss.
A manual transmission has a disadvantage in that it must be frequently shifted by a driver as the number of shift stages increases. In the automatic transmission, however, a control unit controls the operation of a power train to shift gears, depending on an operation state. Thus, attempts have been continuously made to implement a power train having a larger number of shift stages.
From this point of view, 4-shift or 5-shift automatic transmissions have been typically used. Recently, however, a 6-shift automatic transmission has been developed and applied to vehicles, and research has been actively conducted to develop 8 or more-shift automatic transmissions for accomplishing higher fuel efficiency.